Lindbloom et al. U.S. Pat. No. 4,436,248 and commonly assigned pending application Ser. No. 784,391 now U.S. Pat. No. 4,678,130 each disclose an apparatus for manually adjusting the shear bar in a forage harvester relative to a rotating cutter head carrying a plurality of knives. In both the patent and the application, adjustment is accomplished by manually turning a first or a second knob to thereby move a first or a second end of a shear bar. In actual practice, adjustment is carried out by turning the knobs and listening for the "tick" as the rotating cutter blades make contact with the shear bar. When the "tick" is heard, the shear bar is backed off until the sound is no longer heard. The adjustment is imprecise, time consuming, and requires that the operator leave his seat to accomplish the adjustment.
German DE No. 33 45 749A discloses the use of a single motor for simultaneously driving linkages connected to both ends of a harvester shear bar to rotating cutter head. Because both linkages are driven by a single motor, and are thus driven equally, it is not possible to automatically adjust the shear bar if it is not initially parallel to the cutter head and parallelism must be effected manually before the automatic adjustment may be accomplished.
In Keeney (WO 82/01299) a potential between a shear bar and a cutter head is monitored and a microprocessor controls a single motor to adjust the shear bar. In this device too, it is necessary that parallelism be first established by manual adjustment.
German OS No. 30 10 416 discloses the use of an acoustic or optical non-contact sensor for monitoring the gap between the fixed and moving blades of a harvester. In this device the sensor monitors the gap between the fixed and moving blades by sensing the proximity of the moving blades and if the gap varies beyond predetermined limits the harvester operation is stopped. No provision is made for utilizing the sensor output to adjust the gap.
In an automatic adjustment system wherein contact between the shear bar and rotating cutter head is sensed to control the adjustment, it is essential that the vibration sensor be in good operating condition. If the sensor or its associated wiring is faulty, or the cutter head is not rotating, the adjustment mechanism may drive the shear bar too far toward the blades of the rotating cutter head thus damaging the shear bar, cutter blades and/or adjustment drive mechanism. In accordance with one aspect of the present invention, the operability of the vibration sensor is repeatedly checked before and during a shear bar adjustment operation and checks are made to insure that the cutter head is rotating at or above a predetermined minimum speed.
Background noise, that is, the vibrations induced by normal harvester operation, is particularly troublesome when vibration sensors are utilized in an automatic shear bar adjustment system. In accordance with a further aspect of the present invention, means are provided for adapting the threshold of the vibration detection system in accordance with the level of background noise so that vibrations resulting from contact between the shear bar and rotating cutter blades may be reliably distinguished from vibrations resulting from normal harvester operation.